Two people decide to hang out without specifically calling it a date. If nothing comes of it, you frame it as just two people hanging out together. If you end up dating, however, you would both point to this event as your first date.

During the Schrodeninger's Date itself, its status in indeterminate. But afterwards when you look back at the event, you can determine what it truly was.

This is derived from the famous Schrödinger's Cat thought experiment in physics. But this has greater opportunity for sexing.

Person: "Andi, was that a first date last night?"
Andi: "I don't know, it was a Schrödinger's Date. He just said that he was bored and wanted to see a movie. I'm not super interested, but if he wanted to go out, I wouldn't mind it and we had a really good time and we'll have to see what comes of it."
vs
Person: "Jase, how was your Schrödinger's Date last night?"
Jason: "It was great, but now we're going to a hockey game this afternoon and then dinner with her parents tonight. I'm pretty sure that's was officially our first date.

Where single guy and single girl, who don't normally spend time alone together, agree to "hang out" and do something fun together for the first time, yet both carefully avoid the word "date". Whether it was in fact a date will be determined only after the fact, once they see how it goes. Until then, the event is a strange quantum superposition, simultaneously both date and non-date. This allows both parties to avoid potential embarrassment.

It could be a date or it could be just a hang out, we won't know till it's over. It's a Schrödinger's Date.